1. Field of the Invention
The present invention relates to a method for producing an electromagnetic wave-shielding material which shields electromagnetic waves generated from front faces of displays such as CRT (cathode ray tube), PDP (plasma display panel), liquid crystal display, EL (electroluminescence) display and FED (field emission display), microwave oven, electronic equipment, printed wiring board and so forth and has transparency, and an electromagnetic wave-shielding material having transparency obtained by the production method.
2. Description of the Background
In recent years, electromagnetic interferences (EMI) have rapidly increased with the increase in utilization of various electric installations and electronics-applied equipments. It is pointed out that EMI not only causes malfunctions and damages to electronic or electric equipments, but also damages health of operators of these apparatuses. Therefore, it is required to suppress intensity of electromagnetic waves emitted from electronic or electric equipments within the range of standards or regulations.
As a countermeasure for the aforementioned EMI, it is necessary to shield electromagnetic waves, and it is evident that the property of metals that they do not transmit electromagnetic waves can be utilized for that purpose. For example, adopted are a method of using a metal body or a highly conductive body as a casing, a method of inserting a metal plate between circuit boards, a method of covering a cable with a metal foil and so forth. However, since an operator needs to recognize characters etc. displayed on a screen of CRT or PDP, transparency of display is required. Therefore, all of the aforementioned methods were unsuitable as a method for shielding electromagnetic waves, since the aforementioned methods often provide opaque display front faces.
In particular, since PDP generates electromagnetic waves in a larger amount compared with CRT or the like, stronger electromagnetic wave shielding ability is desired. The electromagnetic wave-shielding ability can be conveniently represented in terms of a surface resistance value. While a surface resistance value of about 300 Ω/sq or smaller is required for a light-transmitting electromagnetic wave-shielding material for CRT, the value of 2.5 Ω/sq or lower is required for a light-transmitting electromagnetic wave-shielding material for PDP, and it is quite necessarily 1.5 Ω/sq or lower, more desirably 0.1 Ω/sq or lower, in a plasma television utilizing PDP as a commercial product. Thus, extremely high conductivity is required.
Further, as for the required level of transparency, the transparency is required to be about 70% or higher for CRT and 80% or higher for PDP, and further higher transparency is desired.
In order to overcome the aforementioned problems, various materials and methods such as those mentioned below have been so far proposed to simultaneously achieve electromagnetic wave-shielding property and transparency by utilizing metal mesh having apertures.
(1) Conductive Fibers
For example, Japanese Patent Laid-open Publication (Kokai) No. 5-327274 discloses an electromagnetic wave-shielding material comprising conductive fibers. However, this shielding material has a drawback that it has a thick mesh line width, and therefore when a display screen is shielded with this shielding material, the screen becomes dark, and thus characters displayed on the display are hard to see.
(2) Electroless Plating-Processed Mesh
A method has been proposed which comprises printing an electroless plating catalyst in a grid pattern by a printing method and then performing electroless plating (for example, Japanese Patent Laid-open Publication Nos. 11-170420, 5-283889 etc.). However, the line width of the printed catalyst is as large as about 60 μm and is not suitable for use in a display that requires a relatively small line width and a precise pattern.
Further, a method is proposed which comprises applying a photoresist containing an electroless plating catalyst, performing exposure and development to form a pattern of the electroless plating catalyst and then performing electroless plating (for example, Japanese Patent Laid-open Publication No. 11-170421). However, the visible light transmittance of the conductive film is 72%, which means insufficient transparency. In addition, since extremely expensive palladium must be used as the electroless plating catalyst, most part of which is removed after the exposure, this method also suffers from a problem of production cost.
(3) Mesh Formed by Etching Utilizing Photolithography
A method has been proposed which comprises forming a thin metal film mesh on a transparent substrate by etching utilizing photolithography (for example, Japanese Patent Laid-open Publication Nos. 2003-46293, 2003-23290, 5-16281, 10-338848 etc.). Since this method enables fine processing, it has advantages that it enables production of a mesh having a high aperture ratio (high transmittance), and the mesh can shield even a strong electromagnetic wave emission. However, this method suffers from problems of the time-consuming and complicated production process and thus high production cost. Moreover, it is known that this mesh has a problem that, since it is produced by etching, the width of the intersectional points in the grid pattern is thicker than that of the straight line portions. Furthermore, the problem of moire is also pointed out, and improvement of this problem has been desired.
Hereafter, the prior art concerning methods of forming a conductive metal silver pattern using a silver salt will be explained.
Light-sensitive materials using a silver salt have conventionally been utilized mainly as materials for recording and circulating images and pictures. For example, they are used for photographic films such as negative color films, black-and-white negative films, films for movies and color reversal films, photographic printing paper such as color paper and black-and-white printing paper and so forth, and emulsion masks (photomasks) and so forth utilizing the fact that a metal silver pattern can be formed as an exposure pattern are commonly used. In all of these, images obtained by exposure and development of a silver salt have values, and the images themselves are utilized.
On the other hand, because developed silver obtained from a silver salt is metal silver, it is considered that the conductivity of metal silver may be utilized depending on the production method. Several proposals utilizing such a principle have been known from old days, and the examples of disclosing specific methods for forming a conductive silver thin film include the followings. In the 1960s, a method for forming a metal silver thin film pattern by a silver salt diffusion transfer process, in which silver is deposited on physical development nuclei, was disclosed in Japanese Patent Publication (Kokoku) No. 42-23746. It is also disclosed that a uniform silver thin film prepared by utilizing a similar silver salt diffusion transfer process and not showing light transmission has a function of decreasing microwaves in Japanese Patent Publication No. 43-12862.
Moreover, a method of utilizing the above principle as it is to conveniently perform light exposure and development using an instant black-and-white slide film and thereby form a conductive pattern is disclosed in Analytical Chemistry, Vol. 72, 645, 2000 and International Patent Publication WO01/51276. Further, a method based on the principle of silver salt diffusion transfer process for forming a conductive silver film that can be used for display electrodes of plasma displays is disclosed in Japanese Patent Publication Laid-open No. 2000-149773.
However, any method has not been known at all for forming conductive metal silver by such methods to shield electromagnetic waves emitted from image display surfaces of displays including CRT, PDP etc. without inhibiting the image display.
In the methods described in the five of references mentioned above, specially prepared physical development nuclei are uniformly provided in a layer in which a conductive metal pattern is formed irrespective of the exposed portion and unexposed portion. Therefore, they suffer from a problem that opaque physical development nuclei remain in an exposed portion in which the metal silver is not formed, and thus light transmission is degraded. The aforementioned problem is serious particularly in a case that the metal pattern material is used as a light-transmitting electromagnetic wave-shielding material for a display such as CRT and PDP.
Moreover, it is also difficult to obtain high conductivity by those methods, and attempts of obtaining a thick silver film for high conductivity should be accompanied by a problem of degradation of transparency. Therefore, even if the aforementioned silver salt diffusion transfer process is used as it is, a light-transmitting electromagnetic wave-shielding material showing superior light transmission and conductivity and thus suitable for shielding electromagnetic waves from image display surfaces of electronic display equipments cannot be obtained.
Further, if the silver salt diffusion transfer process is not used, and conductivity is imparted by using a commercially available usual negative film through development, physical development and plating processes, obtainable conductivity and transparency are insufficient for use as a light-transmitting electromagnetic wave-shielding material for CRT or PDP.
As described above, any method has not been known at all for forming a conductive metal silver from a silver salt light-sensitive material as means for shielding electromagnetic waves emitted by electronic display equipments, and if the known silver salt diffusion transfer process is used as it is for electromagnetic wave shielding of displays, transparency and conductivity becomes insufficient.
As described above, conventional electromagnetic wave-shielding materials and methods for producing them each have problems.
In recent years, an electromagnetic wave-shielding plate obtained by forming a mesh composed of a metal thin film on a transparent glass or plastic substrate surface, in particular, has been increasingly used as an electromagnetic wave-shielding material for display panels such as PDP, since it has extremely high electromagnetic wave-shielding property and favorable light transmittance. However, since its price is extremely high, reduction of the production cost has been strongly desired. Furthermore, since high lightness of images is required for a display, light transmittance close to 100% has been required. However, when the aperture ratio (ratio of portions not having fine lines constituting a mesh with respect to the total area) is increased in order to improve the light transmittance, conductivity becomes lower, and thus electromagnetic wave-shielding effect is degraded. Therefore, it has been extremely difficult to simultaneously improve conductivity (electromagnetic wave-shielding effect) and light transmittance with conventional techniques. Moreover, since the material is installed on the front of the image display surface of a display, moire is generated, and it has constituted a problem.